1. Field of the Invention
The present invention relates to a method for surface treatment of carbon fibers for reinforcing mortar or concrete.
2. Description of the Prior Art
Carbon fibers are easily available and are excellent in such mechanical characteristics as specific strength, inelastic ratio and chemical resistant. They have thus enjoyed extensive use. However, they are seldom employed as they are but are generally used as a reinforcing material.
Mortar or concrete reinforced with carbon fibers is mainly employed in roofs, walls, floors, etc. in the fields of civil and architectural engineering. Thus improvement of the bonding strength between the carbon fibers and cement is highly desired.
Hitherto, attempts have been made to mix cement system composite materials with carbon fibers and various improved characteristics have been reported in this connection by, for example, in M. A. Ali, A. J. Majumdar and D. L. Rayment "Carbon-fiber Reinforcement of Cement" (Cement and Concrete Research, vol. 2, pp 201-212, 1972), Akihama (Annual Report of Kajima Construction Technology Institute, vol. 29, 1981), and others.
Various modifications have been made with respect to the form, shape, etc. of carbon fibers, but carbon fibers without sizing or as treated with epoxy resin etc. have generally been employed.
However, in such cases the bonding strength between the carbon fibers and the cement matrix is low and accordingly is not sufficient to ensure the reliability of the mortar or concrete reinforced thereby. Thus the full strength of the carbon fibers has not been fully utilized.
Since carbon fibers have poor bonding property with respect to a cement matrix, it has been impossible to make full use of the strength of carbon fibers in mortar or concrete reinforced thereby. For this reason, various methods for surface treatment of carbon fibers have been studied.
However, when carbon fibers which have not been sized or whose surface has been oxidized are employed as a reinforcing material, the cement matrix does not permeate into the carbon fiber strand and only the carbon fibers on the outside of the strand make contact with the cement matrix. As a result, the bonding between the carbon fiber strand and the cement matrix is poor.
Furthermore, while sizing carbon fibers using colloidal silica (SiO.sub.2) may improve the wettability of the carbon fibers with respect to the cement matrix, the cement matrix does not permeate into the strand so that bonding between the carbon fibers and the cement matrix is not improved.
Still further, while the method of bundling carbon fibers with epoxy resin may help to transmit stress to the filaments constituting the strand, the bonding strength between the epoxy resin and cement matrix nevertheless remains poor.
In addition, it has been pointed out that since carbon fibers have high strength and high elasticity but do not exhibit an appreciable yield characteristic, there exists a danger of breakage due to sudden fracture of the carbon fibers.